Isoprene, the 2-methyl analogue of 1,3-butadiene, has a high production volume and is used largely in the manufacture of synthetic rubber. Isoprene is also the major endogenous hydrocarbon exhaled in human breath. Two-week and 13-week inhalation toxicology studies were conducted in male and female F344/N rats and B6C3F1 mice to characterize potential adverse effects of isoprene. Male rats and male mice were also exposed to isoprene vapors for 6 months followed by a 6-month recovery period (stop-exposure protocol) to determine if isoprene produces a carcinogenic response similar to that of 1,3-butadiene after intermediate exposure durations. In addition to histopathology, evaluations included clinical pathology, tissue glutathione analyses, forelimb and hindlimb grip strength analyses, and sperm motility and vaginal cytology. Data from inhalation teratology studies of isoprene in rats and mice are also reported. In vitro genetic toxicity studies included assessments of mutagenicity in Salmonella typhimurium and sister chromatid exchanges and chromosomal aberrations in Chinese hamster ovary cells. In conjunction with the inhalation studies in mice, evaluations were also made of sister chromatid exchanges and chromosomal aberrations in bone marrow cells and micronuclei in peripheral blood of male mice exposed to isoprene for 12 days or 13 weeks.
Target concentrations of isoprene in the inhalation chambers were 0, 438, 875, 1,750, 3,500, and 7,000 ppm in the 2-week studies; 0, 70, 220, 700, 2,200, and 7,000 ppm in the 13-week and stop-exposure studies; and 0, 280, 1,400, and 7,000 ppm in the teratology studies. In the 2-week studies, no changes related to chemical administration were observed in survival, body weight gain, clinical signs, hematologic or clinical chemistry parameters, or the incidence of gross or microscopic lesions in rats. In mice, there were no effects on survival; the mean body weight of males in the 7,000 ppm group was less than that of the controls. In mice, exposure to isoprene caused decreases in hematocrit values, hemoglobin concentrations, and erythrocyte counts, atrophy of the testis and thymus, cytoplasmic vacuolization of the liver, olfactory epithelial degeneration in the nasal cavity, and epithelial hyperplasia in the forestomach.
Exposure to isoprene for 13 weeks produced no discernible toxicologic effects in rats. In the stop-exposure study, interstitial cell hyperplasia of the testis was observed in all male rats in the 7,000 ppm group after 6 months of exposure. Following the 6-month recovery period, male rats exposed to 700, 2,200, or 7,000 ppm isoprene had slightly greater incidences of interstitial cell adenomas of the testes than the controls.
Exposure to isoprene for 13 weeks or 6 months produced no clear exposure-related effects on body weight gain in male or female mice; however, survival was decreased for male mice exposed to 7,000 ppm isoprene for 6 months. More notably, toxic and carcinogenic effects were induced at multiple organ sites in mice exposed to isoprene. After 6 months of exposure and 6 months of recovery, male mice exposed to 700 ppm or higher concentrations of isoprene had greater incidences of neoplasms of the liver (0 ppm, 7/30; 70 ppm, 3/30; 220 ppm, 7/29; 700 ppm, 15/30; 2,200 ppm, 18/30; 7,000 ppm, 17/28), lungs (2/30, 2/30, 1/29, 5/30, 10/30, 9/28), forestomach (0/30, 0/30, 0/30, 1/30, 4/30, 6/30), and harderian gland (2/30, 6/30, 4/30, 14/30, 13/30, 12/30) than the controls. In addition to the higher neoplasm incidences in male mice exposed to 700 ppm or greater, incidences of multiple neoplasms and/or neoplasms of greater malignancy were also higher than in the controls. Hematologic effects similar to those occurring in exposed mice in the 2-week study, plus greater mean cell volume values than in the controls, were observed after 24 days and after 13 weeks of exposure to isoprene. These hematologic effects, which were not accompanied by greater reticulocyte counts or a higher frequency of polychromatic erythrocytes than controls, were indicative of a nonresponsive, macrocytic anemia. In male mice in the stop-exposure study, partial hindlimb paralysis in the 7,000 ppm group and a dose-related decrease in grip strength were observed near the end of the 6-month exposure period. Other nonneoplastic effects in mice exposed to isoprene included spinal cord and sciatic nerve degeneration, skeletal muscle atrophy, degeneration of the olfactory epithelium, epithelial hyperplasia of the forestomach, increased estrous cycle length, testicular atrophy, and decreased epididymal weight, sperm head count, sperm concentration, and sperm motility. The inhalation teratology studies did not show maternal or developmental toxicity in Sprague-Dawley rats at exposures of up to 7,000 ppm isoprene; in CD-1® Swiss mice, exposure to isoprene resulted in lower fetal weights and a higher percentage of fetuses per litter with supernumerary ribs.
Isoprene was not mutagenic in Salmonella typhimurium and did not induce sister chromatid exchanges or chromosomal aberrations in Chinese hamster ovary cells with or without exogenous metabolic activation; however, in mice, isoprene induced increases in the frequency of sister chromatid exchanges in bone marrow cells and in the frequency of micronucleated erythrocytes in peripheral blood.
These inhalation studies showed that isoprene caused toxic effects in the testis of rats and at multiple organ sites in mice. In F344/N rats, exposure to 7,000 ppm isoprene for 6 months caused an increase in the incidence of testicular interstitial cell hyperplasia, and after 6 months of recovery there was a marginally increased incidence of benign testicular adenomas that may have been related to isoprene administration. No-observable-adverse-effect levels (NOAELs) for isoprene-induced toxic lesions in mice were:
- 70 ppm for nonresponsive, macrocytic anemia, decreased hindlimb grip strength, olfactory epithelial degeneration, and decreases in epididymal weights, spermatid head counts, sperm concentration, and sperm motility;
- 220 ppm for forestomach epithelial hyperplasia;
- 700 ppm for increased estrous cycle length;
- and 2,200 ppm for testicular atrophy, sciatic nerve degeneration, and muscle atrophy.
A NOAEL was not achieved for spinal cord degeneration (less than 70 ppm) or developmental toxicity (less than 280 ppm, based on lower body weights of female fetuses). In addition, the 6-month inhalation exposure plus 6-month recovery (stop-exposure) study provided clear evidence of carcinogenicity of isoprene in the liver, lung, forestomach, and harderian gland of mice. Because these studies involved exposures of male rats and male mice to isoprene for only 6 months, they do not necessarily reveal the full carcinogenic potential of isoprene in these species. Most of the toxic and carcinogenic effects seen with isoprene were also caused by inhalation exposure to 1,3-butadiene.